A Universal Serial Bus (USB) is known as one of interfaces between a host unit such as a personal computer and a device or unit such as a printer unit. Further, a parallel interface is known as one of interfaces which carry out data transmission between a host unit such as a personal computer and a device or unit such as a printer unit. Here, there is a case in which a parallel interface is used at the printer unit, and a USB interface is used at the host unit such as a personal computer, and an interface apparatus which interfaces these units is known.
For example, a prior art document (Jpn. Pat. Appln. KOKAI Publication No. 2002-14913) discloses an interface circuit which interfaces a USB interface and a parallel interface. In this publication, when GET_PORT_STATUS is received from a personal computer via the USB interface, a port status is transmitted to the personal computer in response to this receipt. In this manner, it becomes possible to send a predetermined processing command or the like to the printer side from the personal computer via the USB interface.
However, in this prior art, for example, in the case where the printer side enters a power saving mode, there is not disclosed how processing should be carried out from the personal computer side in order to enter a normal mode. Therefore, there is a problem that the personal computer side cannot cause the printer unit to carry out smooth printing processing reliably via the USB interface and parallel interface.